Induction of an immune response requires T cell expansion, differentiation, contraction and establishment of T cell memory. T cells must encounter antigen presenting cells (APCs) and communicate via T cell receptor (TCR)/major histocompatibility complex (MHC) interactions on APCs. Once the TCR/MHC interaction is established, other sets of receptor-ligand contacts between the T cell and the APC are required, i.e. co-stimulation via CD154/CD40 and CD28/B7.1-B7.2. The synergy between these contacts is suggested to result, in vivo, in a productive immune response capable of clearing pathogens and tumors, and in some cases capable of inducing autoimmunity.
Another level of control has been identified, namely regulatory T cells (Treg). This specific subset of T cells is generated in the thymus, delivered into the periphery, and is capable of constant and inducible control of T cells responses in vitro and in vivo (Sakaguchi (2000) Cell 101(5):455-8; Shevach (2000) Annu. Rev. Immunol. 18:423-49; Bluestone and Abbas (2003) Nat. Rev. Immunol. 3(3):253-7). Treg are represented by a CD4+CD25+ phenotype and also express high levels of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), OX-40, 4-1BB and the glucocorticoid inducible TNF receptor-associated protein (GITR) (McHugh, et al. (2002) Immunity 16(2):311-23; Shimizu, et al. (2002) Nat. Immun. 3(2):135-42). Elimination of Treg cells by 5 day neonatal thymectomy or antibody depletion using anti-CD25, results in the induction of autoimmune pathology and exacerbation of T cells responses to foreign and self-antigens, including heightened anti-tumor responses (Sakaguchi, et al. (1985) J. Exp. Med. 161(1):72-87; Sakaguchi, et al. (1995) J. Immunol. 155(3):1151-64; Jones, et al. (2002) Cancer Immun. 2:1). In addition, Treg have also been involved in the induction and maintenance of transplantation tolerance (Hara, et al. (2001) J. Immunol. 166(6):3789-3796; Wood and Sakaguchi (2003) Nat. Rev. Immunol. 3:199-210), since depletion of Treg with anti-CD25 monoclonal antibodies results in ablation of transplantation tolerance and rapid graft rejection (Jarvinen, et al. (2003) Transplantation 76:1375-9). Among the receptors expressed by Treg, GITR seems to be an important component since in vitro or in vivo ligation of GITR on the surface of Treg with an agonistic monoclonal antibody results in rapid termination of Treg activity (McHugh, et al. (2002) supra; Shimizu, et al. (2002) supra), also resulting in autoimmune pathology (Shimizu, et al. (2002) supra) and ablation of transplantation tolerance.
DNA microarray analysis has been conducted with a population of Treg to identify genes differentially expressed by Treg (Gavin, et al. (2002) Nat. Immunol. 3(1):33-41; McHugh, et al. (2002) supra). The expression pattern of genes of CD4+CD25- and CD4+CD25+ T cells was compared (Gavin, et al. (2002) supra) as was the expression pattern of these two populations of cells after activation by anti-CD3 antibody and IL-2 for 12 and 48 hours (McHugh, et al. (2002) supra). However, gene regulation by GITR signaling was not assessed.
T cell activation is dependent upon signs transferred through antigen-specific T cells receptor recognition and accessory receptors on the T cell. As the maintenance of immunologic peripheral homeostatis is regulated by co-stimulatory molecules, which play a critical role in suppressing autoreactive lymphocytes, identification of these co-stimulatory molecules and co-inhibitory ligands is needed.
Costimulatory and co-inhibitory ligands and receptors not only provide a “2nd signal” for T cell activation, but also a balanced network of positive and negative signal to maximize immune responses against infection while limiting immunity to self. The best characterized costimulatory ligands are B7.1 and B7.2, which are expressed by professional APCs, and whose receptors are CD28 and CTLA-4 (Greenwald, R. J., Freeman, G. J., and Sharpe, A. H. (2005). Annu Rev Immunol 23, 515-548; Sharpe, A. H., and Freeman, G. J. (2002) Nat Rev Immunol 2, 116-126). CD28 is expressed by naïve and activated T cells and is critical for optimal T cell activation. In contrast, CTLA-4 is induced upon T cell activation and inhibits T cell activation by binding to B7.1/B7.2, thus impairing CD28-mediated costimulation. CTLA-4 also transduces negative signaling through its cytoplasmic ITIM motif (Teft, W. A., Kirchhof, M. G., and Madrenas, J. (2006). Annu Rev Immunol 24, 65-97; Teft, W. A., Kirchhof, M. G., and Madrenas, J. (2006). Annu Rev Immunol 24, 65-97. B7.1/B7.2 KO mice are impaired in adaptive immune response Borriello, F., Sethna, M. P., Boyd, S. D., Schweitzer, A. N., Tivol, E. A., Jacoby, D., Strom, T. B., Simpson, E. M., Freeman, G. J., and Sharpe, A. H. (1997)) Immunity 6, 303-313; Freeman, G. J., Borriello, F., Hodes, R. J., Reiser, H., Hathcock, K. S., Laszlo, G., McKnight, A. J., Kim, J., Du, L., Lombard, D. B., and et al. (1993). Science 262, 907-909), whereas CTLA-4 KO mice mice can not adequately control inflammation and develop systemic autoimmune diseases (Chambers, C. A., Sullivan, T. J., and Allison, J. P. (1997) Immunity 7, 885-895; Tivol, E. A., Borriello, F., Schweitzer, A. N., Lynch, W. P., Bluestone, J. A., and Sharpe, A. H. (1995) Immunity 3, 541-547; Waterhouse, P., Penninger, J. M., Timms, E., Wakeham, A., Shahinian, A., Lee, K. P., Thompson, C B., Griesser, H., and Mak, T. W. (1995). Science 270, 985-988.
The B7 family ligands have expanded to include costimulatory B7-H2 (ICOS Ligand) and B7-H3, as well as co-inhibitory B7-H1 (PD-L1), B7-DC (PD-L2), B7-H4 (B7S1 or B7x), and B7-H6 Brandt, C. S., Baratin, M., Yi, E. C., Kennedy, J., Gao, Z., Fox, B., Haldeman, B., Ostrander, C. D., Kaifu, T., Chabannon, C., et al. (2009) J Exp Med 206, 1495-1503; Greenwald, R. J., Freeman, G. J., and Sharpe, A. H. (2005) Annu Rev Immunol 23, 515-548.
Inducible costimulatory (ICOS) molecule is expressed on activated T cells and binds to B7-H2 Yoshinaga, S. K., Whoriskey, J. S., Khare, S. D., Sarmiento, U., Guo, J., Horan, T., Shih, G., Zhang, M., Coccia, M. A., Kohno, T., et al. (1999). Nature 402, 827-832. ICOS is important for T cell activation, differentiation and function, as well as essential for T-helper-cell-induced B cell activation, Ig class switching, and germinal center (GC) formation Dong, C., Juedes, A. E., Temann, U. A., Shresta, S., Allison, J. P., Ruddle, N. H., and Flavell, R. A. (2001) Nature 409, 97-101; Tafuri, A., Shahinian, A., Bladt, F., Yoshinaga, S. K., Jordana, M., Wakeham, A., Boucher, L. M., Bouchard, D., Chan, V. S., Duncan, G., et al. (2001) Nature 409, 105-109; Yoshinaga, S. K., Whoriskey, J. S., Khare, S. D., Sarmiento, U., Guo, J., Horan, T., Shih, G., Zhang, M., Coccia, M. A., Kohno, T., et al. (1999) Nature 402, 827-832. Programmed Death 1 (PD-1) on the other hand, negatively regulates T cell responses. PD-1 KO mice develop lupus-like autoimmune disease, or autoimmune dilated cardiomyopathy depending upon the genetic background Nishimura, H., Nose, M., Hiai, H., Minato, N., and Honjo, T. (1999) Immunity 11, 141-151. Nishimura, H., Okazaki, T., Tanaka, Y., Nakatani, K., Hara, M., Matsumori, A., Sasayama, S., Mizoguchi, A., Hiai, H., Minato, N., and Honjo, T. (2001) Science 291, 319-322. The autoimmunity most likely results from the loss of signaling by both ligands PD-L1 and PD-L2. Recently, CD80 was identified as a second receptor for PD-L1 that transduces inhibitory signals into T cells Butte, M. J., Keir, M. E., Phamduy, T. B., Sharpe, A. H., and Freeman, G. J. (2007) Immunity 27, 111-122. The receptor for B7-H3 and B7-H4 still remain unknown.
B7-H6 is a newly identified B7 family ligand, which binds to an activating receptor NKp30 on natural killer cells in humans Brandt, C S., Baratin, M., Yi, E. C., Kennedy, J., Gao, Z., Fox, B., Haldeman, B., Ostrander, C D., Kaifu, T., Chabannon, C., et al. (2009). J Exp Med 206, 1495-1503.
The two inhibitory B7 family ligands, PD-L1 and PD-L2, have distinct expression patterns. PD-L2 is inducibly expressed on DCs and macrophages, whereas PD-L1 is broadly expressed on both hematopoietic cells (i.e. T cells, DCs, B cells, macrophages, and mesenchymal stem cells) and non-hematopoietic cell types (i.e. endothelial cells, pancreatic islet cells, and muscle cells) Keir, M. E., Butte, M. J., Freeman, G. J., and Sharpe, A. H. (2008) Annu Rev Immunol 26, 677-704; Okazaki, T., and Honjo, T. (2006). The PD-1-PD-L pathway in immunological tolerance. Trends Immunol 27, 195-201. Consistent with the immune-suppressive role of PD-1 receptor, studies using PD-L1−/− and PD-L2−/− mice have shown that both ligands have overlapping roles in inhibiting T cell proliferation and cytokine production Keir, M. E., Liang, S. C., Guleria, I., Latchman, Y. E., Qipo, A., Albacker, L. A., Koulmanda, M., Freeman, G. J., Sayegh, M. H., and Sharpe, A. H. (2006). Tissue expression of PD-L1 mediates peripheral T cell tolerance. J Exp Med 203, 883-895. PD-L1 in particular, contributes significantly to tempering the development of autoimmunity and promoting peripheral tolerance Keir, M. E., Butte, M. J., Freeman, G. J., and Sharpe, A. H. (2008). PD-1 and its ligands in tolerance and immunity. Annu Rev Immunol 26, 677-704. PD-L1 deficiency enhances disease progression in both the NOD model of autoimmune diabetes and the murine model of multiple sclerosis (EAE) Ansari, M. J., Salama, A. D., Chitnis, T., Smith, R. N., Yagita, H., Akiba, H., Yamazaki, T., Azuma, M., Iwai, H., Khoury, S. J., et al. (2003). The programmed death-1 (PD-1) pathway regulates autoimmune diabetes in nonobese diabetic (NOD) mice. J Exp Med 198, 63-69; Fife, B. T., Guleria, I., Gubbels Bupp, M., Eagar, T. N., Tang, Q., Bour-Jordan, H., Yagita, H., Azuma, M., Sayegh, M. H., and Bluestone, J. A. (2006). Insulin-induced remission in new-onset NOD mice is maintained by the PD-1-PD-L1 pathway. J Exp Med 203, 2737-2747; Latchman, Y. E., Liang, S. C., Wu, Y., Chernova, T., Sobel, R. A., Klemm, M., Kuchroo, V. K., Freeman, G. J., and Sharpe, A. H. (2004). PD-L1-deficient mice show that PD-L1 on T cells, antigen-presenting cells, and host tissues negatively regulates T cells. Proc Natl Acad Sci USA 101, 10691-10696; Salama, A. D., Chitnis, T., Imitola, J., Ansari, M. J., Akiba, H., Tushima, F., Azuma, M., Yagita, H., Sayegh, M. H., and Khoury, S. J. (2003). Critical role of the programmed death-1 (PD-1) pathway in regulation of experimental autoimmune encephalomyelitis. J Exp Med 198, 71-78.
PD-L1−/− T cells produce elevated levels of the pro-inflammatory cytokines in both disease models. In addition, the tissue expression of PD-L1 uniquely contributes to its capacity of regionally controlling inflammation. Studies in NOD mice have demonstrated that PD-L1 expression on hematopoietic cells alone is insufficient to prevent autoimmune diabetes. Instead, its expression within pancreas is critical for protection against self-reactive CD4+ T cells Keir, M. E., Liang, S. C., Guleria, I., Latchman, Y. E., Qipo, A., Albacker, L. A., Koulmanda, M., Freeman, G. J., Sayegh, M. H., and Sharpe, A. H. (2006). Tissue expression of PD-L1 mediates peripheral T cell tolerance. J Exp Med 203, 883-895. PD-L1 is also highly expressed on placental syncytiotrophoblasts, which critically control the maternal immune responses to allogeneic fetus Guleria, I., Khosroshahi, A., Ansari, M. J., Habicht, A., Azuma, M., Yagita, H., Noelle, R. J., Coyle, A., Mellor, A. L., Khoury, S. J., and Sayegh, M. H. (2005). A critical role for the programmed death ligand 1 in fetomaternal tolerance. J Exp Med 202, 231-237.
Consistent with its immune-suppressive role, PD-L1 potently suppresses anti-tumor immune responses and aids tumor escape from immune surveillance. PD-L1 can induce apoptosis of infiltrating cytotoxic CD8+ T cells, which express high level of PD-1 (Dong, H., and Chen, L. (2003). J Mol Med 81, 281-287; Dong, H. et al., (2002), Nat Med 8, 793-800). Studies in murine tumor models have shown that blocking the PD-L1:PD-1 signaling pathway, in conjunction with other immune therapies, prevents tumor progression by enhancing anti-tumor CTL activity and cytokine production Blank et al., (2004), Cancer Res 64, 1140-1145; Blank et al., (2005), Cancer Immunol Immunother 54, 307-314. Blank et al., (2006). Int J Cancer 119, 317-327; Geng et al., (2006), Int J Cancer 118, 2657-2664; Iwai et al., (2002), Proc Natl Acad Sci USA 99, 12293-12297. Iwai et al., (2005). Int Immunol 17, 133-144. Moreover, a recent study by the inventors shows that PD-L1 expression on DCs promotes the induction of adaptive Foxp3+CD4+ regulatory T cells (aTregs), and PD-L1 is a potent inducer of aTregs within the tumor microenvironment (Wang et al., (2008), Proc Natl Acad Sci USA 105, 9331-9336. Disruption of this pathway by PD-L1 mAb or PD-L1 KO mice reduced tumor-mediated induction of aTregs and reduced tumor growth.
Based on the foregoing, the elucidation of other novel B7 type family members and ligands and modulators thereof would be useful given the role of these family members in regulating immunity.